Image amplifier



Oct. 7, 1941.

P. T. FARNSWORTH IMAGE AMPLIFIER Filed April 5, 1939 INVENOIL PH/LO r. FARNSWORTH.

BY M W ATTORNEYS.

Patented Oct. 7, 1941 r;

, IMAGE AMPLIFIER,

' I RhiloT, Farnsworth, Philadelphia, Pa., assignor fto Farnsworth Television & Radio Corporation,

W New York, N. Y., a corporation ofDelav r are 7 Application April 5, 1939,,seria1;Nb.,'2t6,1as"

My, invention relates to'image amplifiers, and more particularly to a means and method of obtaining a train of electrical signals representing an optical image. My; system is, particularly adapted for use in television; I v

Among the objects of my; invention are: To provide an image amplifying tube of high sensitivity and fidelity; to provide animage amplifyingtube and system for use in television transfacing toward the electron gun. Betweenthecup Band the electron gun is awall film: I'l attached by a connection l2 to. the anode ,1. of the gun.

Wall Coating l approaches but does not connect -5 withcupHl;

mitters; to provide an'image amplifying system 1'0 Both of thesescreens l iand I;5-ar e preferably of utilizing grid control of electron emission from elemental cathodes; to provide a means and method of producinga charge storage in an image amplifying tube; to provide a'means and method extremely fin wir in. der, that they: doin t greatly affect an opticalimageprojected by lens 4 on charge storage electrode 9,;

Positioned betweenscreens Hand l5 'is a offorming a charge storage electrode; to provide r 5 second wall coating [(5 electrically separate from ;a means and method of controlling the storage {time of a'charge storage electrode; to provide a charge storage electrode adapted'for use in an image amplifyingtelevision pickup system; to

either screen, and atone side of the tube con necting with th space between screens it and J5, a multiplier tubulation l1, 'Ihismultiplier tubulation-I'I supports-and positionsa multiprovide an image amplifying tube having a signal 1'20 plier shell I 9 havingan opening-20 angular1y facoutput well; above inherent interference values; and to provide a. means and method of operating a charge storage image amplifying tube and system. I 1

ing screen I4 and charge storage electrode 9r, u not extendinginto the optical-path,

-I may desire to utilize as an output-devicean electron multiplier, preferably ofthebox element My invention possessesfnumerous other ;ob-

:type, such as described 1 and claimed in L the jectsandfeaturesof advantage, some of which, [together with theforegoing, will be set-forth in the following de'scriptionof specific apparatus embodying and utilizing my novel method. It is Richard L. Snyder application for United States Letters Patent entitled Box element multiplied, Serial N0. 149,65fi, filed June 22, 1937, now United States Patent No. 2,163,966 issuedJune 27-, 1939.

therefore to be understood. that my method is ,The first 'threestages are cube-shaped silver applicable toother apparatus,' and-thatI do not limit myself, in any way, to theapparatus of the present application, asI may adoptvvarious other apparatus embodiments, utilizing the method,

within the'scope of the appended claims.

Referring to the drawing: v g

Fig. 1 is a longitudinal sectional view, partly diagrammatic, showing one preferred form of my invention, together with a circuit by which the tube may be set into operation. f r V Fig. 2 is a perspective view, partly in section, showing one preferred form of charge storage electrode. j

, Referring directly to Fig.; 1,for.-a description boxesa -Onefaceof= each stage is open, and an 5 The'open side of first-stageZ l is.presented to the tungsten screen of second stage-24,- and in similar fashion the open'end of second stage 24 is presented to the screened side of third stage 25. An 1 output screenflis positioned across the open ggft -end-of third stage 25, and is backed bylast stage of a preferred form of my invention shown thereclaimed in the Snyder patent cited abovaiu. I

in, an envelope-1 is-provided at,. one-end,thereof with aplanar-window-i through which light from an object may, be projected by ,a'lens At, the opposite end of the tube is, positionedan electron .Before describing the connections ,oftthel yarious. elements lin thetube shown. in Fig. 1,v I will -describe,-by reference toFig. 2,;the structur a'nd formation of charge storage electrode 9. While gun. which may be of conventionalgdesign, ,utilizfi I have found that various'wireV-meshscreens may v-ing an indirectlyheated thermionic cathode 5, .,a control grid, 6 andanaccelerating, anode l, Be-

tween the electrcn gun and the planar window 2, and coaxial therewith, is haree,s cra e-. c-

sbeutilized for the. foundation of this charge storage electrode, I- have found it preferable to .utilize a perforated nickel sheet,.30 -having approximately -160,000. perforations 3 l-, persquare rodev gmgunted in a. cup IO the rimof the cup 5 inch, and approximatelye0-.0005inch thick.- {The placing barium fluoride in a container approxi- While there is another step, usually found desirable in the formation of the charge storage electrode 9, namely, the step of controlling the leakage time between the photoelectric mosaic 34 and the nickel screen 30, it is believed that a description of one type of circuit utilized to place the tube in operation will render the discussion of this final step more understandable. Accordingly, cathode 5 may be connected to a heating circuit in a manner well known in the art, in order that this cathode emit electrons. Grid 6 is connected by wire 40 through blocking conmately 6 inches from the screen, and heating the container. 9

I have found that a film of dielectric between 0.0003 and 0.0005 inch thick maybe evaporated onto the nickel sheet in about five minutes. This process provides the dielectric coating 32, with the edges of this coating slightly overlapping the edges of the perforations in the nickel sheet, as shown in the sectional views of Fig. 2. I have also found that it is desirable to apply a tickler coil to the nickel sheet during the evaporation process in order to prevent too great an accumulation of barium fluoride on the sides of the perforations. Bulging sides at these points will greatly reduce the size of the openings in the sheet, which is, of course, undesirable.

The next step is to sputter a film of silver, indicated by globules 34 in Fig. 2, on top of the barium fluoride coating 32, preferably in an atmosphere of rarified oxygen. This silver film should preferably have extremely low conductivity approaching infinite resistance, and it has been found that less conductivity may be obtained by using oxygen than with any other gas.

The nickel screen with its dielectric and silver coatings may then be mounted in envelope I, with the silvered surface facing the planar window 2 and with the nickel surface facing the electron gun. The other elements are then placed within the envelope l and the tube is degassed by baking for several hours at a high temperature. The silver film 34 is then oxidized in a glow discharge in the usual manner, well known in the art.

After cooling, the temperature is reduced to 200 C'., and caesium is slowly distilled into the envelope 1 between the gun and the charge storage electrode 9 until the direct current photoelectric peak of the sensitized silver is just passed. The tube is then baked at 200 C. and baked for a short time until the peak value of photoemission is again reached. This procedure results in a mosaic photoelectric surface on the side of the charge storage electrode 9 facing planar window 2 and lens 4.

It will be obvious, however, that I have described just above only one method of obtaining a mosaic photoelectric surface, and I do not wish to be limited in any way to this particular method, as other methods and other materials may also be used to obtain the same result, as is well known in the art.

After the charge storage electrode 9 has been formed as just above described, the electron multiplier stages 2 I, 24, 25 and 26 may then be'treated with suitable materials in order to obtain therefrom secondary electron emission at ratios greater than unity. caesium treatment similar to that required to produce photoelectric surfaces .may 'be applied to the multiplier stages directly through side tubulation J1 without affecting the sensitization of the charge storage-electrode9.

denser 4| to a signal input to which blanking impulses may be applied to extinguish the cathode ray beam'during retrace intervals, and grid 6 is maintained at the proper biasing potential by bias battery 42 and bias resistor 44. Anode 1 is energized by anode source 45 at ground potential, source 45 placing the cathode 5 more negative than ground potential. Scanning oscillators 49 and 41 through scanning coils 49 and 59 move the electron beam emitted from anode I in two directions over a picture area on the nickel side of charge storage electrode 9, and focusing coil 48 is used to define the beam. The nickel screen 30 forming the foundation of this charge storage electrode is made more positive than anode 1 by means of a tap 5| on voltagedividing resistor 52fthis resistor being energized by the source 54. Multiplier housing l9 and connected film l6 are energized at the next higher potential by tap 55; accelerating screen I4 is held at the next higher potential by tap 56; first multiplier stage 2| is held at the next higher potential by tap 51; second multiplier stage 24 is held at the next higher potential by tap 58; third multiplier stage 25 is held at the next higher potential by tap 59; last stage 26 is held at the next higher potential by tap 69.

Output screen 21 of the multiplier is held at the highest potential. through a final output circuit comprising a filter resistor 6|, a choke coil 62 and an output resistor 63. Output screen 21 is also connected through a blocking condenser 62" to the control grid 63 of an output tube 64, the latter having a bias resistor 65, a screen bias connection 61, and an output resistor 68 connected to the anode 69 of output tube 64, all in the manner well known in the art for the connection of a screen-grid tube. Other output circuits are fully equivalent.

In operation the electron gun may be energized, for example, with a voltage of 500 to 1500 volts, to bombard the nickel side of the charge storage electrode 9. Some of the electrons in the gun beam, as it is scanned over the picture area on the nickel side of charge storage electrode 9, will .pass through the perforations of the electrode and are collected on end screen l5 which is only slightly positive relatively to the gun anode I, as indicated by end screen tap 69 on voltage divider 52. About of the electrons in the gun beam, however, strike the metallic nickel surface of electrode 9 to produce secondary electrons.

I prefer to have a potential difference of from 5 to 50 volts positive on the accelerating screen l'4 relatively to the nickel base of the charge storage electrode 9. This potential is insufficient, however, to draw any of thesecondaryelectrons created by beam impact from the nickel side of the Storage electrode 9 through the-perforations of the storage electrode. However, when an optical image ,is focused on the mosaic side of the grid through a'l'ens '4, photoelectrons are emitted and drawn to screen Mileaving the mosaicf'ele ments on thezphotoelectrici sideotstora'ge electrode 9 charged positivelyilwith' :aI'ch'arge that. in.- creases proportionally to the product-of light in.- tensity and time. This additional positive charge results in.secondary'electrons'from the nickel side being'pulled through the perforations in an amount which is'proportionalto thethree-halves power'of the voltage between the nickelsurface and the photoelectric surface. It should be noted here. that the control of. each. mosaic. elementv is relativelylarge, since this mosaic element, acting as a grid, is only about one-thousandth of. an inchfrom the source "of the secondary electrons. The signal electrons: are. accelerated through screen. I4 into thespacebetween screens. and I5 and are drawninto th multiplier. for multiplication therein. r a v vWelthus have a multiplicity 'of small amplifiersof which only thoseevery near the area bombarded by the scanning gun beam have an emitting cathode. The current in the gun beam need not be more than a few microamperes, since this is'quite: adequate to give space charge saturationpfor if a beam of elemental size is used and then multiplied by the number of picture elements, the effective cathode emission for the total number of elements 'may be on the order of an ampere or more. 4

i .It will be noted that the elemental'gridelements, though efiectively. free under the conditions described thus far, are operated positively. Itmight be expected, therefore, that the grid current drawn by it would be excessive, and

that the sensitivity would be decreased due to the rapid discharge of a grid element when the scanning beam was operating in its vicinity. Almost the opposite has been foundto be true, however, and if a high potential difference is applied between screen l4 and the charge storage electrode 9. the beam discharge action'is very low.- Thus, the tube may be operated tohave a memory or storage time of from fivetoten minutes, thus rendering the tube practically useless for the transmission of movingeobjectsi but useful ior other. purposes. I 1

Reducing. the voltage applied between the screen I4 and the grid improves the/speed of response and naturally decreases the sensitivity accordingly; In practice, the voltagediiference is adjusted to be as high as possible without blurring of the field when the object moves. It is not necessary, vhowever,that the elemental mosaic grid elements be operated'free, as the leakage time may be controlled by evaporating the thin film of metal through the perforations in the grid from the nickel side until the time constants of the grid have the desired value. This effectively results in'providing the grid ele-' ments around the'perforations with a high-resistance leak across the edges of the insulating layer 32 at the edges of the perforations. In this manner the leakage timemay be controlled as desired. If desired, a small nickel filament may be positioned 'within the gun end of the tube and utilized during initial operation of the tube to control the leakage time. Other metals, such as magnesium or the like, may be used.

It should be noted that the potential of screen M is higher than the potential of multiplier shield l9 and attached wall film I6, but that the potential of first stage 2|, presenting its screen 22 to the space between screens I4 and I5, is the highest positive potential acting on this space.

' Consequently, signal electrons passing through screen I 4 are directed toward the multiplier put, pass through opening-ZEl and' first-stage} screen 22, and. impact the sensitized surface of the first stage 21. Secondary electrons emitted. by impact in first stage ZIare' acceleratedinto second stage 24 to. create 'more secondary elec-* trons. These secondary electrons are accelerat ed into third stage Z5 tocreate further electrons; and these latter are then accelerated toimp'act last stage 26. Secondary electron s emitted from the last stage 26 are collectedby output screen 27, and the output current, comprisingthe signal current multiplied by repeated liberation of secondary electrons at a ratio greater than unity, is thereupon utilized -inthe output. circuit in the usual manner for whatever purpose-desired. It has been found, experimentally, that signal electrons from all portionsof the charge storage electrode 9 can be uniformly collected in-"thefirst stage of the multiplier withan' efiiciency oflapproximately i It should be pointed out alsothat one'of the features of the present invention is the practical elimination of noise or irregularity' 'in-output due to the electrons otthe-scanningbear'n passing through the charge storageelectrode 9, When no means are provided for collecting these primary-beam electrons" to keep them from flowing in the output circuit,- iti's'necessarythat the component of 'signalcurrent be at least- 19% of the total beam current. Thus-," if"the beam current were to be 5 microamperes, the-signal component would haveto be at least 0.5-microampere; in other words, the sensitivity of 'the tube would be reduced approximately- 10 to-jl; However, by keeping primary 'electrons out" of the work circuit by the useflo'f the electronmuh tiplier and the collection of scanning-beam electrons on screen l5, this reduction insensitivity is entirely eliminated. Furthermore, the fluctu ation voltage present on the elemental grid ele ments, that is produced by both thermal agitation and shot irregularities of the photocurrent, has a maximum frequency --component in the neighborhood of ten cycles per second, and "this frequency component is not passed by the exter nal' video-frequency amplifiers. Also, there is little or no shot noise inthe multipliedcurrenti The electron multiplier, therefore, may be uti lized to increase the sensitivity of thetube by a factor of from twenty to one hundred times, notvv only without increasing the"-nois' factorsybut with. actual reductionthereof-because of the elimination of the scanning b'ea m" electrons. from the output. 5 The merits of the image amplifying tube and; system described and claimed herein may there fore be summed up as follows: (1) 7 It provides a short optical system with the image plane perpendicular'to the axis of'the lens; (2) No excessive power is required in the scanning beam; "q *j I f (3) The electron beam sc'ar'isthe cathode p'e'r pendicularly; i1 1 (4) There is no quired; and (5) My inventionproducsa lightsensitivity several orders better than'previous types of ele'ctronic scanning devices without increase in noise level.

I claim:

in ompensation r f;

1. An image amplifier system comprising an envelope containing an electron-permeable charge storage electrode comprising a conducting founfor scanning successive elementary areas of the 1 impacted side of said electrode; means spaced from said, electrode to collect beam electrons passing through said electrode; and separate means. for collecting secondary electrons passing through said permeable electrode.

2. Apparatus in, accordance with claim 1, with an acceleratingscreen positionedzin,,front of said mosaic and energized" to a potential insu.flicient to draw secondary electrons through saidgpermeable electrode in the absence of light on said mosaic.

3. Apparatus in. accordance with claim 1, with means positioned onthe side of said permeable electrode remotefrom said electron beam produoing means for collecting photoelectrons emitted from said mosaic surface.

4. Apparatus in accordance with claim 1, wherein the separate means for collecting the secondary electrons passing through said permeable electrode is an 'output electrode positioned between said mosaic and the means for collecting beam electrons, and positioned outside of the path of said beam electrons passing through said permeable electrode.

5. Apparatus in accordance with claim 1, wherein the separate means for collecting the secondary electrons passing through said permeable'electrode is the first stage of an electron '1 multiplier, said first stage being positioned outside of 1 the path of beam electrons passing through said permeable electrode.

6. Apparatus in accordance with claim 1, with an accelerating screen positioned in front of said mosaic andenergized to a potential insufficient to draw secondary electrons through said permeable electrode in the absence of light on said mosaic but energized to a potential sufficient to collect electrons emitted from said mosaic in the presence of light, said separate means for collecting secondary electrons being positioned between said accelerating screen and said means for collecting beam electrons and out of the path of beam electrons passing through said permeable electrode.

'7.v An image amplifier system comprising an envelope containing an electron-permeable charge storage electrode comprising a conducting foundation, a layer of insulatingmaterial on one. side only of said electrode, the other side ofsaid electrode being capable of producing secondary electrons at a ratio greater than unity, and a,

mosaic photoemissive surface on said insulating material; means for focusing an optical image on said mosaic; electron producing and directing means for developing an electron beam of elemental cross-section impacting the secondaryemissive side of said electrode at a velocitysufficint. to; produce secondary electrons therefrom; means forscanningi: successive elementary areas of 'the impactedside ofsaid electrode; anaccelerating screen positioned. parallel to and adjacent said mosaic, a beam-electron collecting screen positioned coaxially with and parallel tosaid permeable electrode and .said accelerating screen and spaced from said accelerating screen; and an output electrode positioned between said collecting screen and said accelerating screen and out of theipath of beam electrons passing through said permeable. screen and said accelerating electrode;

8. Apparatus in accordance with claim '7, wherein said output electrode is the first stage of" anelectron multiplier energized to a higher potential than either said accelerating screen or said beam-electron collecting screen to attract secondary electrons passing through said permeable electrode and said accelerating screen.

9. In a system utilizing an electron-permeable electrode having one side capable of producing secondary electrons at a ratio greater than unity and an opposite side carrying a photosensitive mosaic spaced from said electrode by an insulating film, the method of producing an electrical signal in response to a light image which comprises generating a primary beam of. electrons of elemental cross-section, directing said beam towards a plane to produce secondary electrons thereat, moving said beam across said plane to produce a cloud of secondary electrons delineating a, picture area, applying an accelerating potential from the other side of said plane insufficient to draw secondary electrons through said plane in the absence of electrical charges therein, creating a charge image in said plane by projecting an optical image thereon and removing photoelectrons emitted thereat, and collecting secondary electrons passing through said plane under the control of charges therein.

10. In a system utilizing an electron-permeable electrode having one side capable of producing secondary electrons at a ratio greater than unity and an opposite side carrying a photosensitive mosaic spaced from said electrodev by an insulating film, the method of producing an electrical signal in response to a light image which comprises generating a primary beam of electrons of elemental cross-section, directing said beam towards a plane to produce secondary electrons thereat, moving said beam across said plane to produce a cloud of secondary electrons delineating a picture area, applying an accelerating potential from the other side of said plane insufiicient to draw secondary electrons through said plane in theabsence of electrical charges therein, creating a charge image in said plane by projecting an optical image thereon and removing photoelectrons emitted thereat, collecting beam electrons passing through said plane, and separately collecting secondary electrons passing through said plane under the control of charges therein.

11. Method in accordance with claim 10, wherein collected secondary electrons are multiplied by repeated secondary electron producing impacts.

PI-HLO T. FARNSWORTH.

CERTIFICATE OF CORRECTION.

Patent No. 2,257 ,9L 2. October 7 19m.

PHILO T. FARNSWORTH.

It is hereby certified that error ap iring correction as follows: Page LL, secof the above numbered patent requ "screen and said ac ond column, lines 12 and 15, claim 7 for the words celerating electrode" read -electrode and said accelerating that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

A. D. 19in.

Signed and sealed this 2nd day of December,

screen-; and

Henry Van Arsdale, Acting Commissioner of Patents.

(Seal) pears in the printed specification CERTIFICATE OF CORRECTION.

Patent No. 2,2 7 ,9L 2. October 7 19m.

PHILO "1. FARNSWORTH.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page it, 560- 0nd column, lines 12 and 15, claim 7 for the words "screen and said accelerating electrode" read electrode and said accelerating soreen; and

that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 2nd day of December, A. D. 19in.

Henry Van Arsdale, Acting Commissioner of Patents.

(Seal) 

